Positive and Negative Geotropism

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We all know that plants can’t walk from place to place, but they do move their stems, leaves, and roots in response to their environment. It just happens too slowly for us to see. These plant movements are called tropisms. In a positive tropism, the plant moves towards the stimulus. One familiar example is positive phototropism, in which a plant moves towards sunlight. Time lapse photography allows us to appreciate the tropisms of plants. This video shows the positive phototropism of sprouting snow peas. Note how the pea sprouts move back and forth with each day.

Plants can also move away from stimulus; this is called a negative tropism. Some plants move away from bright light, this tropism is called negative phototropism

Other physical phenomena can stimulate plant movement. Geotropism is the movement of a plant in response to gravity. This is especially evident in the early sprouting of bean seeds.

In this investigation, you will investigate how gravity affects the sprouting of radish seeds. Try to determine if gravity causes a positive or negative response in the plant.

Problem: How does gravity affect the growth of sprouting seeds?

Materials

Ruler

Aluminum foil

Scissors

3 Paper towels

Spray bottle

Water

Radish seeds

Labels or masking tape

Permanent marker

Box big enough to fit a cookie sheet on the bottom

Cookie sheet or other large pan

Masking tape

Dark room

Dropper

Procedure

Using the ruler and scissors, measure and cut three 12”x12” squares of aluminum foil.

Take a paper towel and fold it in half twice.

Set the folded paper tower in the middle of one piece of aluminum foil.

Using the spray bottle, spray the paper towel until all of it is moist, but not dripping wet.

Make a line of about twenty radish seeds through the center of the paper towel.

Carefully fold each side of the paper towel over the seeds in the center, covering them. What do you think the paper towel is for?

Next, fold the foil around the paper towel. Why is the foil needed along with the paper towel?

Place a label on the foil packet.

Write “1: Bottom” on the label.

Rest one end of the cookie sheet on top of the cardboard box so that the cookie sheet creates a ramp.

Place the labeled foil envelope on top of the cookie sheet.

Make two more foil packets filled with the same amount of seeds.

Label the second packet “2: Side”.

Tape this packet on the side of the box.

Label the third packet “3: Turn”.

Tape this packet to another side of the box.

Every other day, rotate the “3: Turn” foil packet 90 degrees on the side of box.

Make sure the paper towels remain moist inside the foil packets. Use a dropper to add a bit more water if necessary.

After five days, carefully observe the sprouting of the seeds, paying careful attention to the direction of stem and root growth in relationship to where the seeds were positioned in the box.

If little growth has occurred or you want to continue your experiment, rewrap your seeds and put them in the same position.

Results

In the “1: Bottom” packet, the stems of the future plant tend to grow upwards, away from gravity. The stems therefore show a negative tropism towards gravity. The roots of the future plants, on the other hand, tend to grow downwards, showing a positive tropism towards gravity. In the “2: Side”.packet, the differing tropisms of roots and stems are likely to be even more evident because of the packet’s vertical orientation.

The seedlings in the “3: Turn” packet might look confused! Each turn reorients the seeds in space, so the stems and roots are likely to show twisting patterns.

Why

The paper towel filled the role of soil, holding water for the sprouting seed. The foil served two purposes: It helped keep the paper towel from drying out while blocking any light stimulus from reaching the seeds. We wanted this experiment to be about geotropism, not phototropism!

How plants perceive and respond to gravity is still a bit of a mystery. It is thought that plants have special cells called statocytes that detect gravity through dense starch grains that tend to settle at the bottom of the root. This settling triggers a chemical message in the plant which in turn causes other chemical messages that make some parts of the plant grow more than others.

Going Further

If tropisms have piqued your interest, you might try designing experiments on other types of tropisms, like phototropism or thigmotropism, which is a plant’s response to touch. The venus fly trap or mimosa would be good experimental candidates.

Tropisms can even be studied in outer space. After seeing the results of your experiment on geotropism, how do you think a seed would sprout in zero gravity?

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